The present invention relates to a paper coating composition containing a pigment and an aqueous binder. More particularly it relates to the paper coating composition imparting excellent printing quality and excellent printing results to paper.
The term "paper" as used herein should be interpreted in its broad sense which includes paper in the narrow sense as well as paperboard.
Coated paper, which is obtained by applying a paper coating composition mainly composed of a pigment and an aqueous binder on paper, followed by necessary steps, such as drying and calendering, has been widely used e.g. for commercial prints, magazines and books due to its excellent properties such as printing results. With the increasing demand for higher quality and the development of high-speed printing techniques, constant efforts have been continued to further improve the quality of coated paper. Particularly in the art of offset printing which predominates in various printing techniques, it is an important subject to improve ink receptivity considering the effects of damping water, water resistance such as wet pick, and anti-blister properties at a rotary press.
In order to solve the above-mentioned problem, it has been known to add wet strength agents or printing quality improvers including melamine-formaldehyde resins, urea-formaldehyde resins and polyamidepolyurea-formaldehyde resins such as those disclosed in, for example, JP-B-69-11667 and JP-B-84-32597 (the term "JP-B" as used herein means an "examined published Japanese patent application (KOKOKU)") to the paper coating composition.
Although these conventional wet strength agents and printing quality improvers exhibit effective characteristics, each of them has a serious defect or insufficiency in part of the desired characteristics and is not always satisfactory for practical use.
For example, aminoplast resins such as melamine-formaldehyde resins and urea-formaldehyde resins, not only cause evolution of formaldehyde during the coating process or from the resulting coated paper but also produce substantially no effect on improving ink receptivity and anti-blister properties. Besides, as the pH of the coating composition increases, the water resistance improving effect by the aminoplast resins becomes less pronounced.
Polyamidepolyurea-formaldehyde resins are effective for improving water resistance, ink receptivity and anti-blister properties. The degree of improvements attainable by them, however, is not necessarily sufficient against the recent demand for higher quality of coated paper. Efforts have hence been made for further improvements. For example, improvements are proposed in JP-A-92-00997 and JP-A-92-263696 (the term "JP-A" as used herein means an "unexamined published Japanese patent application (KOKAI)"). Nevertheless, there still has been need for further improved performance to cope with the ever increasing demand for coated paper quality.
An object of the present invention is to provide a paper coating composition which can cope with the demand and gives coated paper higher qualities such as higher water resistance and ink receptivity than those obtainable by conventional techniques.
The present inventors have conducted extensive investigation; as a result, have found that a paper coating composition containing a mixture or a reaction product of a specific water-soluble resin and a specific amide or aminonitrile compound can give excellent properties to paper; and have thus attained the present invention.